A piezoelectric ceramic constitutes an electronic device that converts electrical energy to mechanical energy, or mechanical energy to electrical energy, based on the principle of piezoelectric effect. Many conventional electronic devices have used lead-containing piezoelectric ceramics constituted by two components of PbTiO3, and PbZrO3 (hereinafter sometimes referred to as “PZT”) or composite perovskite piezoelectric ceramics combining this PZT with the third component such as Pb(Mg1/3Nb2/3)O3 or Pb(Zn1/3Nb2/3)O3.
However, these electronic devices contain Pb as a main component and therefore present problems in terms of high environmental burdens generated from volatilization of PbO in the production process, for example. This gives rise to a need to develop piezoelectric, ceramics free from Pb or containing less Pb.
Examples of Pb-free piezoelectric ceramics include, among others, a composition comprising a perovskite structure constituted by BaTiO3 (refer to Non-patent Literatures 1 and 2), a composition of perovskite structure containing a bismuth constituted by two components of (Bi1/2Na1/2)TiO3 and (Bi1/2K1/2)TiO3 (refer to Patent Literatures 1 to 4), a composition comprising a tungsten bronze structure whose main component is (Ba, Sr, Ca)2NaNb5O15 (refer to Patent Literatures 5 to 7), a composition comprising a bismuth layer structure whose main component is SrBi2Nb2O9 (refer to Patent Literatures 8 to 10), and a composition comprising an alkali-containing niobate-type perovskite structure whose main component is KNbO3—NaNbO3—LiNbO3 (refer to Patent Literatures 11 to 13). Among others, the piezoelectric ceramics whose main component is KNbO3, particularly, offer relatively high piezoelectric characteristics and are expected to replace lead-containing piezoelectric ceramics.
With piezoelectric sensors such as acceleration sensors, impact sensors and knock sensors, the higher the voltage generated in response to the input mechanical stress such as acceleration, impact or pressure, the better the sensor sensitivity becomes. Accordingly, piezoelectric ceramics used to constitute these sensors should desirably have as high an electromechanical coupling coefficient (such as k31) as possible and as low a dielectric constant (such as ∈33T/∈0) as possible. In general, electric charge C that generates when mechanical stress is given to piezoelectric ceramics becomes higher when the electromechanical coupling constant is higher. Also, electric charge C is proportional to the product of dielectric constant ∈ and voltage V (in other words, the relationship of C∝∈ V holds). Accordingly, voltage V that generates is proportional to C/∈ if the electromechanical coupling constant and the acceleration added by mechanical stress are both constant (in other words, the relationship of V∝C/∈ holds), meaning that the lower the dielectric constant ∈, the higher the generated voltage becomes. In the case of an acceleration sensor, it is desirable to use piezoelectric ceramics whose mechanical quality coefficient (Qm) is relatively high. When the mechanical quality coefficient is high, energy loss occurring in the ceramics can be kept low. The response, therefore, increases, and consequently the heat generation caused by continuous acceleration, impact or pressure can be suppressed. As explained above, piezoelectric ceramics used for acceleration sensors, etc., should ideally have a high electromechanical coupling coefficient, low dielectric constant, and high mechanical quality coefficient.
Patent Literature 14 disclosed that when CuO is added to piezoelectric ceramics whose main component is KNbO3, the dielectric constant can be lowered and mechanical quality coefficient raised without lowering the electromechanical coupling coefficient. Also in Non-patent Literatures 3, 4 and Patent Literature 15, ceramics comprising a tungsten bronze structure, such as K4CuNb8O23, K5Cu2Nb11O30, K5.4Cu1.3Ta10O29, are proposed as additives comprising the same effect as CuO.